[libc] Fix install-libc to work with LLVM_LIBC_FULL_BUILD=OFF (#197366)
Initialize variables that are conditionally set to avoid undefined
references in install-libc and install-libc-stripped targets:
- Initialize added_bitcode_targets to empty string (may be undefined
when LIBC_TARGET_OS_IS_GPU=OFF)
- Initialize startup_target to empty string and only set to
"libc-startup" when both LLVM_LIBC_FULL_BUILD=ON and NOT baremetal
(startup directory is only included in full builds)
- Initialize header_install_target to empty string (may be undefined
when LLVM_LIBC_FULL_BUILD=OFF)